Electric Heating
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Introduction
Modes of Transfer of Heat
Classification of Electric Heating Methods
Resistance Heating
Arc Furnaces
Induction Heating
Dielectric Heating
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Heating is required for domestic purposes as
well as industrial purposes.
Industrial applications include
Melting of Metals
Hardening
Tempering
Case Hardening
Drying
Melting
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Economy
Cleanliness
Absence of Flue Gases
Ease of Control
Automatic Protection
Upper Limit of Protection
Special Heating Requirement
High Efficiency of Utilization
Better Working Conditions
Safety
1.
Conduction
2.
Convection
3.
Radiation
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In this mode of transfer of heat, one molecule
of the substance gets heated and transfer the
heat to the adjacent and so on.
Rate of conduction of heat along a substance
depends upon temperature gradient.
Q=kA/t(T1-T2)T
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Convection is the movement of molecules
within fluids (i.e. liquids, gases).
It cannot take place in solids, since neither bulk
current flows nor significant diffusion can take
place in solids.
Radiation is a process in which energetic
particles or energy or waves travel through a
medium or space.
It can be classified as:
 Direct Resistance Heating
 Indirect Resistance Heating
 Infra-red or Radiant Heating
 Arc Heating
 Direct Induction Heating
 Indirect Induction Heating
 Dielectric Heating
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It is based upon the I R effect.
Has wide applications in heat treatment of
metals, drying, baking of potteries, stoving of
enamelled ware and domestic cooking.
Temperature upto 1000 celsuis can be obtained
in ovens.
Two common methods of resistance heating
are
Direct Resistance Heating
Indirect Resistance Heating
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It is accomplished by the dissipation of an
electric current in a material.
Material to be melted is placed inside a furnace
with electrodes in the walls.
Alternating or direct electric current is then
passed between the electrodes through the
material.
Resistance losses are dissipated as heat, causing
the material to melt.
Advantages:
 High efficiency.
 It gives uniform heat and high temperature.
Application:
 It is mainly used in salt bath furnace and water
heaters.
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In this method the current is passed through a
highly resistance element which is either
placed above or below the oven depending
upon the nature of the job to be performed.
The heat proportional to I2R losses produced in
heating element delivered to the charge either
by radiation or by convection.
Both A.C and D.C supplies can be used for this
purpose at full mains voltage depending upon
the design of heating element.
Application:
 This method is used in room heater, in
bimetallic strip used in starters, immersion
water heaters and in various types of resistance
ovens used in domestic and commercial
cooking.
It must posses the following properties:
 High Resistivity
 High Melting Point
 Low Temperature Coefficient
 Free Form Oxidation
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An electric arc furnace is a furnace that heats
charged material by means of an electric arc.
There are two common types of arc furnaces.
Three phase furnace.
Single phase furnace.
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Three phase furnaces are used in the
production of alloy steels.
Single phase furnaces are used for the
manufacture of gray iron casting also.
Three phase furnaces are used for power
ratings from 250KVA, 10,000KVA and
capacities up to 25 tons.
Generally graphite electrodes are used. As they
are subjected to volatilization, they are to be
replaced.
The arc temperature is between 3000 and
3500˚C, so that the process is carried out
between 1500˚C and 2500˚C.
1)Variable ratio power transformer
2)Reactors
3)Automatic current regulator
4)Control panel
5)Electric motor and tilting motor
6)Circuit breaker and connecting switches.
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The arc is struck directly with the charge, when
a current flows through it and produces intense
heat, which results, in high temperature.
Although some furnaces up to 100 tons are
made, generally furnaces up to 25 tons are in
general use.
Stirring action is automatic and gives a uniform
product. It is used for alloy steel manufacture
and gives a purer product.
Advantages:
 It produces purer products
 It is very simple and easy to control the
composition of the final product during
refining process.
Disadvantages:
 It is very expensive.
Application:
 The most common application of this type of
furnace is to produce steel.
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Electrodes are inserted from the sides and the
heat produced is transmitted by radiation to
the charge.
As there is no inherent stirring action, the
furnace should be rocked.
This furnace is used for only single phase
supplies. Also the capacity of the furnace is
limited up to 100 tons.
Melting of non-ferrous metals is mostly carried
out in this type of furnace.
In both the type of furnaces, large quantities of
electrodes are used.
Application:
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The main application of this type furnace is
melting of non-ferrous metals.
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It consists of an iron core, crucible and primary
winding connected to an AC supply.
The charge is kept in the crucible, which forms
a single turn short circuited secondary circuit.
The current in the charge is very high in the
order of several thousand amperes. The charge
is magnetically coupled to the primary
winding.
The charge is melted because of high current
induced in it. When there is no molten metal,
no current will flow in the secondary.
To start the furnace molten metal is poured in
the oven from the previous charge.
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The magnetic coupling between the primary
and secondary is very weak, therefore the
leakage reactance is very high. This causes low
power factor.
Low frequency supply is necessary because
normal frequency causes turbulence of the
charge.
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It is modified type of core type induction furnace.
It has a vertical channel for the charge, thus the crucible used is
also vertical.
The principle of operation is that of a transformer in which the
secondary turns are replaced by a closed loop of molten metal.
The primary winding is placed on the central limb of the core.
Hence leakage reactance is comparatively low and power factor is
high.
The top of the furnace is covered with an insulated cover which
can be removed for charging. Necessary arrangements are usually
made for titling the furnace to take out the molten metal.
When primary is connected to the AC supply, high current will be
accumulated at the bottom and even a small amount of charge will
keep the secondary completed.
Hence chances of discontinuity of the circuit is less.
Advantages:
 High efficiency and low operating cost.
 Since both primary and secondary are on the
same central core, its power factor is better.
 The furnace is operated from the normal
supply frequency.
Applications:
 This furnaces is used for melting non ferrous
metals like brass, zinc, tin, bronze, copper etc.
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In such furnace an inductively heated element
is made to transfer its heat to the change by
radiation.
It consists of an iron core linking with the
primary winding and secondary.
When the primary winding is connected to the
supply, current is induced in the secondary of
the metal container.
So heat is produced due to induced current.
This heat is transmitted to the charge by
radiation.
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Coreless induction furnace also operates on the
principle of transformer. In this furnace there is
no core and thus the flux density will be low.
Hence for compensating the low flux density,
the current supplied to the primary should
have sufficiently high frequency.
Stirring of the metals takes place by the action
of the electromagnetic forces. Coreless furnace
may be having conducting or non conducting
containers.
To prevent the primary winding from high
temperature, refractory linings are provided
between primary and secondary windings.
Advantages:
 Time taken to reach the melting temperature is
less.
 Accurate power control is possible.
 Any shape of crucible can be used.
 The eddy currents in the charge results in
automatic stirring.
 Absence of dirt, smoke, noise, etc.
 Erection cost is less.
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Dielectric heating is also sometimes called as
high frequency capacitance heating.
If non metallic materials i.e, insulators such as
wood, plastics, china clay, glass, ceramics etc
are subjected to high voltage A.C current, their
temperature will increase in temperature is due
to the conversion of dielectric loss into heat.
The dielectric loss is dependent upon the
frequency and high voltage. Therefore for
obtaining high heating effect high voltage at
high frequency is usually employed.
Advantages:
 Uniform heating is obtained.
 Running cost is low.
 Non conducting materials are heated within a
short period.
 Easy heat control.
Applications:
 For food processing.
 For wood processing.
 For drying purpose in textile industry.
 For electronic sewing.
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